1. Field
The present disclosure generally relates to the fabrication of composite stiffeners, and deals more particularly with a method and apparatus for reducing wrinkling of contoured hat-type stiffeners formed from a single composite charge.
2. Background
Composite stiffeners such as spars and stringers are used in a wide range of aerospace, marine and other applications. In some cases, stiffeners must be contoured along their length to conform to the geometry of a structure, such as an aircraft wing or fuselage skin.
One technique for fabricating contoured composite stiffeners involves punch forming a composite laminate charge into a straight stiffener, and contouring the straight stiffener by placing a contourable vacuum fixture inside the stiffener. Vacuum is applied to the vacuum fixture and the stiffener is then transferred to a female cure tool having a desired stiffener contour. This approach is labor intensive and adds to tooling costs.
More recently, it has been proposed to fabricate contoured composite stiffeners, such as stringers, using punch forming in which a flat composite laminate charge is punched formed into a die to achieve the desired cross-sectional shape. Both the punch and the die are articulated along their lengths, allowing them to bend. Once the charge has been punch formed to the desired cross-sectional shape, the punch and the die are forced to bend, thereby forming the stiffener to the desired contour. Problems arise, however, where the composite stiffener is highly contoured. As the formed charge is bent to the desired contour, portions of the charge, such as the webs in a hat stiffener, are placed in compression, resulting in undesired wrinkling of some of the plies of the charge. Hand rework of the stiffeners is required to eliminate the wrinkling, adding to labor costs and reducing production rate.
Accordingly, there is a need for a method and apparatus that allow punch forming of highly contoured composite stiffeners, such as hat stringers, which substantially reduce or eliminate ply wrinkling caused by material compression as the stiffener is formed to the desired contour.